1. Field of the Invention
The present invention relates to a communication technology that uses a MIMO (Multi-Input Multi-Output) antenna system, and especially relates to an adaptive modulation technology of suitably adjusting a transmission rate according to a state of a communication link in the communication system using MIMO.
2. Description of the Related Art
As a technology of realizing radio transmission at high speed and quality using a limited frequency band, there is an adaptive modulation technology. The adaptive modulation technology adaptively sets up a transmission rate (the number of modulation levels, encoding rate, etc.) of a transmitting signal according to the state of a transmission line. For example, low speed QPSK is used when the receiving state of an electric wave is poor, and high-speed 16QAM is used when the receiving state is good. Further, adaptive control is performed as for encoding according to the propagation state (receiving state) such that encoding with a large error correction capability is used when the receiving state is poor, and encoding with a small error correction capability is used when the receiving state is good.
As an index indicative of the state of the transmission line, a ratio of received signal power to interference and noise (SINR), and a Doppler frequency, and the like are used. A method of realizing high-quality communication is known wherein the transmission rate of a signal is determined in consideration of the receiving state (for example, Non-Patent Reference 1).
FIG. 1 shows the configuration of a publicly known adaptive modulation type transceiver that includes a transmitter 100 and a receiver 200. According to the conventional adaptive modulation transmission and reception, a signal is transmitted and received using a pair of antennas, one being by the transmitter and the other being by the receiver. The transmitter 100 includes an antenna 101, a CRC bit adding unit 102, and an encoder/mapper 103 such that reliability information, such as CRC (Cyclic Redundancy Check), is added to a bit sequence of an input information signal. The signal to which CRC is added is then encoded, modulated, and transmitted from the antenna 101. By the receiver, SINR of the transmission line is estimated by a channel estimating unit 202. Further, based on SINR, the received signal is demodulated, and decoded by a detector 203 and de-mapper/decoder 204, respectively. A CRC error detecting unit 205 performs error detection of the decoded signal. A transmission rate determining unit 206 determines the number of modulation levels and the encoding rate of the transmitting signal based on the SINR value and error detection results that are obtained.
FIG. 2 is a flowchart that shows a transmission rate determination process of the conventional transceiver in which adaptive modulation is performed. First, a communication process is started at Step S1001, then a receiving frame number t and a SINR margin are initialized, and an increment width “delta_up” of the SINR margin and a decrement width “delta_down” of the SINR margin are set up.
Next, the SINR value of the transmitted signal is calculated by channel estimation at Step S1002. Further, at Step S1003, detection and decoding of the transmitting signal are performed using the channel estimate, and error detection is performed using CRC from the decoding result. At Step S1004, the SINR margin is obtained based on the error detection result at S1003 and the estimated SINR at S1002. When there is an error in the decoded result (CRC is NG), the SINR margin value is increased by “delta_down”, and when the decode result is error-free (CRC is OK), the SINR margin value is decreased by “delta_up”.
Next, at Step S1005, the SINR margin value is subtracted from the SINR value, and the transmission rate (the number of modulation levels, the encoding rate, etc.) is determined using the subtracted result and a look-up table. When the communication quality is degraded and an error is detected due to influence of Doppler change of the transmission line, and the like, the transmission rate is determined based on the estimated SINR less the margin value. In this way, precision of transmission rate determination is raised. The look-up table used at Step S1005 is a correspondence table of SINR and transmission rates (the number of modulation levels, the encoding rate, etc.) for a given FER (frame error rate), and can be prepared in advance. At Step S1006, the determined transmission rate is fed back to the transmitter 100. This process is repeated until the communication ends (YES at S1007).
The adaptive modulation described with reference to FIGS. 1 and 2 is an adaptive modulation between the transmitter and the receiver, each having a single antenna. On the other hand, in a case where the mobile communications system uses the MIMO method, i.e., two or more antennas, a method of switching and controlling communication operations of space paths based on the communication quality information of space paths wherein a reception error is detected, is proposed (for example, Patent Reference 1). By this method, in order to prevent signal interference from arising in the paths between two or more antennas, and the connection from being lost, the reception error of two or more space paths is detected, and communication operations of the space paths are suitably switched between two or more kinds of operations based on an error detection result. Specifically, the first kind of operation is disconnecting all paths except for one path, the second kind of operation is performing path diversity, and the third kind of operation is lowering the modulation factor of a path.
[Non-Patent Reference 1] NEC and Telecom Modus, “Selection of MCS levels in HSDPA”, 3GPP TSG RAN WG1 Technical Document, R1-01-0589, May 2001
[Patent Reference 1] JPA 2003-244045